Lamps of the above-mentioned type comprise a reflector through the central region of which the lamp's base is passing. The glass bulb of the lamp itself has a seal stud at its end remote from the reflector. This constructive arrangement affects, however, the emission characteristic of the lamp in a disadvantageous manner in that the light intensity is significantly smaller in the central region than in regions situated more off-side the optical axis. If light emitted from such a reflector lamp is guided to an object to be illuminated, for example, through a fiber optical light guide, the same distribution characteristic of light intensity will, in principle, appear at the exit surface of this light guide. The maximum solid angle of light emission at light's exit from the light guide will be limited precisely by the numerical aperture of the glass fiber.
The solid or special angle of light emission of reflector lamps described above amounts about to 70.degree., and glass fiber light guides of 3 mm, 5 mm or 8 mm are used, for example, to guide the light to the object to be illuminated. The dark central spot occurring due to the constructive design of the reflector lamp described above will appear at the exit of the light guide the more significantly, the smaller the active diameter of the bundle of the fiber optic light guide is. This disadvantageous effect is utterly spoiling, for instance, when illuminating an object to be examined by microscope. Such a brightness distribution is especially disadvantageous with light guides comprising fiber bundles which run in common on the light entrance side, but are separated at the light exit side, i.e. which are then divided into a plurality of light guides.
In order to avoid a dark central spot, it has already been suggested to arrange the reflector lamp inclined with respect to the axis of the light entrance surface of the light guide. This, however, results merely in a size reduction of the dark central spot. Even with an inclined lamp, the above-mentioned dark spot appears, some times due to tolerances in manufacture of the reflector lamp. Moreover, a special drawback will occur with small active diameters of the light guide, even with an inclined lamp, in that the improvement in brightness distribution in the central region of an illuminated surface is rather insignificant.
Furthermore, it has been suggested to arrange a wedge-like optical element in the region between the reflector lamp and the entrance surface of the light guide in order to avoid a dark central spot. Although this known suggestions results in an improvement in brightness distribution in the central region, the average light intensity over the surface to be illuminated will be reduced significantly by such a measure. The reason for this disadvantageous effect is that, although some light intensity is transferred into the central angular region by wedge-like elements, light is also directed concurrently to the light guide in an angular region which lies outside its numerical aperture and, therefore, cannot be transferred any further.